Photoconductive compositions and electrophotographic recording elements made therefrom



United States Patent 3,428,452 PHOTOCONDUCTIVE COMPOSITIONS ANDELECTROPHOTOGRAPHIC RECORDING ELEMENTS MADE THEREFROM Edward C. Giaimo,Jr., Princeton, N.J., assignor to Radio Corporation of America, acorporation of Delaware No Drawing. Filed Jan. 18, 1965, Ser. No.426,443 US. Cl. 961.7 Int. Cl. G03c J 74 ABSTRACT OF THE DISCLOSURE Animproved photoconductive layer is applied to a substrate of anelectrophotographic record from an aqueous coating mixture comprisingparticles of water insoluble resin-encapsulated dye-sensitized zincoxide dispersed in an aqueous mixture comprising a resinous bindingagent for said particles. The encapsulation of the dyesensitized zincoxide prevents desorption of the water solu- 'ble light-sensitizing dyesin the aqueous coating mixtures.

This invention relates generally to photoconductive compositions whichare particularly useful in electrophotography. Particularly, theinvention relates to improved photoconductive layers, containingdye-sensitized photoconductors, and improved methods of making theselayers from aqueous coating solutions.

By the term aqueous coating solution, as used herein, is meant adispersion of a dye-sensitized photoconductor in water in which aresinous binding agent is either dissolved or dispersed. Although thedispersion of the dyesensitized photoconductor is not a true solution,the term solution is nevertheless herein used because this term has beenadopted by the prior art.

It has been proposed to render certain photoconductive materials, suchas the photoconductive layer of an electrophotographic recordingelement, for example, responsive to light in the visible spectrum bycausing certain dyes, such as auramine, acridine orange, and rhodamineB, to be adsorbed to the photoconductive layer. Other suitable dyes arelisted in US. Patent No. 3,052,540, issued to H. G. Greig on Sept. 4,1962. Since, however, most lightsensitizing dyes in common use are verysoluble in water, the photoconductive layer, a white body coloredphotoconductor, such as zinc oxide, has heretofore been deposited from acoating solution that comprises a lightsensitizing dye, zinc oxide, anda resin binder for the zinc oxide in a liquid vehicle other than water.Organic solvents, such as toluene and methyl ethyl ketone, for example,in which light-sensitizing dyes are substantially insoluble, have beenused heretofore as the liquid vehicle for photoconductive coatingsolutions to prevent desorption of the dye into the liquid. Compared toaqueous coating solutions, however, organic solvent coating solutionsare relatively expensive, dangerous to handle, and usually present afire hazard in a factory or paper mill.

Where water has been used heretofore as the liquid vehicle coatingsolutions, the dyes desorbed by the water would tend to objectionablycolor the substrate of an electrophotographic recording element to whichthe photoconductive coating was applied, leaving uneven colored streaks.Because of this objectionable desorption of the dyes in water, anexcessive quantity of dyes had to be used, resulting in an excessivewaste of the dyes and uneven sensitization. Also, it has often beenimpossible to obtain the same degree of light sensitization of thephotoconductive layers from prior art aqueous coating solutions as hasbeen obtained from non-aqueous coating solutions because of theobjectionable desorption of the dyes in water.

14 Claims 1 It is an object of the present invention to provide anImproved photoconductive layer deposited on a substrate from arelatively inexpensive and safe aqueous coating solution.

. Another object of the present invention is to provide improved methodsof producing photoconductive compositions employing an aqueous solutionof dye-sensitized photoconductive particles whose dyes are not easilydesorbed in aqueous coating solutions.

Still another object of the present invention is to provrde improvedphotoconductive layers and improved methods of making them that overcomethe aforementioned disadvantages of employing light-sensitizing dyes inprior art aqueous coating solutions.

Still a further object of the present invention is to provide improvedphotoconductive layers that may be applied, by the improved methods ofthe present invention, directly, safely, and economically to a web ofsubstrate record material, such as paper, during its manufacture.

Briefly stated, the improved photoconductive layers of the presentinvention are applied to suitable subtrates from aqueous coatingsolutions containing particles of completely encapsulated dye-sensitizedzinc oxide dispersed in an aqueous film-forming emulsion of a naturaland/or synthetic resin. The aqueous resin emulsion functions as afilm-forming vehicle and, after drying, functions as a resinous bindingagent for the encapsulated dye-sensitized particles. Each of theencapsulated dye-sensitized particles comprises a core of aphotoconductive material such as zinc oxide, at least onelight-sensitizing dye adsorbed to the photoconductor, and asubstantially waterproof resin encapsulating the dye photoconductor.

The particles of encapsulated dye-sensitized photoconductor can beproduced, in a preferred method of the present invention, by firstmaking a homogeneous mixture of photoconductive zinc oxide, a solutionof at least one light-sensitizing dye, and a solution of a resin in asuitable solvent for the resin and then spray-drying the homogeneousmixture under conditions to form encapsulated particles of a desiredaverage diameter.

The novel features of the present invention, both as to its organizationand operation, as Well as additional objects and advantages thereof,will be more readily understood from the following detailed description.

Encapsulated dye-sensitized zinc oxide particles of the type describedherein will be hereinafter referred to as EDSZO. The quantitative ratiosbetween the zinc oxide, the dyes and the encapsulant therefor may varywithin wide limits depending upon the degree of light sensitizationdesired and/or the coating solutions in which the EDSZO is to beincluded. For example, the weight ratio of dyed zinc oxide to resinencapsulant may be as small as 2:1 and as great as 7.5 :1. The weightratio of dyed zinc oxide to the total resin in the aqueous coatingsolution may be as little as 2:1 and as great as 3:1. In fact, particlesof EDSZO can be fused directly onto a backing substrate, without beingdeposited from a solution, to form a suitable photoconductive layer.

One method of preparing EDSZO in accordance with the present inventionis:

EXAMPLE 1 A concentrated stock solution of a light-sensitizing dyemixture is prepared by making a solution of 0.5 g. of Brilliant Blue FCF(42090 Color Index Acid Blue 9- 2nd ed. 1957) and 0.01 g. of fluoresceinin 197.5 of ethanol.

42 g. of photoconductive zinc oxide, 2 ml. of the concentrated dyemixture, and ml. of methanol are mixed in a ball mill for 2 hours. Thedyed zinc oxide then is filtered and dried.

A one percent by weight solution of Pliolite SD resin in toluene isprepared.

The dried dyed zinc oxide is mixed with the one percent 'Pliolite S-SDresin solution in toluene in the proportions of 5 g. of the dried diedzinc oxide for each 45 ml. of the Pliolite 'S-SD solution in a ball millfor 2 hours. The milled mixture is spread thinly on glass and thetoluene is evaporated leaving dried, died zinc oxide encapsulated in thePliolite S5D resin. The dried, encapsulated dyed zinc oxide is scrapedoff the glass surface and then ground in a mortar to a fine powder ofparticles averaging about 1 micron in diameter. The quantities ofingredients of Example 1 may vary so that the Weight ratio of the zincoxide to the encapsulating resin may be within the range of 27.5: l.

The photoconductive zinc oxide used in this and the following examplesmay, for example, be either Florence Green Seal No. 8 or Photox 801,products of the New Jersey Zinc Company, Palmerton, Pa. Pliolite S-SD isa synthetic resin of the high styrene butadiene copolymer type, aproduct of the Goodyear Tire and Rubber Company. EDSZO made pursuant toExample 1 is photoconductive in response to wavelengths of light overthe entire visible spectrum.

The encapsulating resin of the dye-sensitized zinc oxide may by the(laboratory) method of Example 1 should be hard and grindable to producea fine powder.

Soft, as well as hard, resinous waterproofing encapsulants for thedye-sensitized zinc oxide may be used to prepare EDSZO by the followingpreferred (commercial) method of Example 2, employing the technique ofspray drying:

EXAMPLE 2 A light-sensitizing dye, Blue Concentrate, is prepared bydissolving 1.2 g. of fiuorescein disodium salt and 3.6 g. of BrilliantBlue FCF in 100 ml. of methanol.

A homogeneous mixture is made of 454 g. of photoconductive zinc oxide, 2ml. of the aforementioned Blue Concentrate, 60 g. to 240 g. (preferably120 g.) of Pliolite S-5E (a high styrene butadiene copolymer resin) and800 ml. of toluene. The homogeneous mixture is then spray dried in aBowen Conical Laboratory Spray Drier (Bowen Engineering, Inc., NorthBranch, NJ.) under the following conditions: Feed (rate 98 cc./min.,air-inlet temperature to drying chamber 208 F., air-outlet temperatureof drying chamber 155 F. Indirect gas heat is used, and a concurrentflow in a two-fluid nozzle atomizer is obtained with an air pressureinput to the nozzle of about 100 lb./in. gage. The aforementionedquantities in Example 2 may vary so that the ratio of zinc oxide to theresin encapsulant may vary from 2 to 75:1 by weight.

The EDSZO formed by the preferred method of Example 2 comprisessubstantially spherical particles of about 1 micron in diameter. Each ofthe particles of EDSZO comprises a core of photoconductive dyesensitizedzinc oxide completely encapsulated by the waterproof resin (PlioliteS5E). The spray drying method (Example 2) of producing EDSZO" ispreferred to the grinding method of Example 1 because it can be adaptedto a closed cycle wherein the solvent of the encapsulating resin may berecovered, if desired. The spray drying method permits the use of a widechoice of both hard and soft encapsulating natural or synthetic resinsbecause the operation of grinding to form a fine powder is obviated.Also, the spray drying method is a practical commercial method,resulting in uniform, substantially spherical, completely encapsulateddye-sensitized, photoconductive zinc oxide particles.

Being encapsulated in a waterprof resin, the EDSZO can now beincorporated in photoconductive coating solulions (dispersions) thathave a water base. The encapsulant on each particle of EDSZO preventsdesorption of the light-sensitizing dyes into the aqueousphotoconductive coating solutions.

An aqueous coating (film forming) solution for forming a photoconductivelayer on a suitable substrate, such as paper, may be prepared asfollows:

EXAMPLE 3 PART A Parts by Ingredients Weight Supplier (grams) EDSZO(prepared in accordance 15.0 (As per Example 2).

with Ex. 2).

PART B 'Iamol #731 (25% aqueous solution) 0.3 Rohm and Haas 00., (sodiumsalt of polymeric carbox- Philadelphia, Pa. ylic acid, a dispersant).Emulphor EL-719 (polyoxyethyl- 0. 1 General Dyestuffs ated fatty acid, adispersant). gorip New York, FMC KP-l40 (plastieizer, leveling 0.05 FoodMachinery agent). ICIorp New York, Methocel 400 cps. (3% aqueous solu-2. 5 Dow Chemical Co.

tion, methyl cellulose). Water 15.0

PART C Piccotex No. 131-119 (a water emul- 9. 0 Penn. Industrial sion ofa styrene homolog copoly- Chemical Corp., met, a resin). Clareton, Pa.Water 15.0 FMC KP- (tributoxy ethyl phos- 0. 5 Food Machinery phate, aplasticizer and leveling Mix together the ingredients of Part B. AddPart A to Part B and mix in a ball mill for about one hour. Mix Part Cingredients together and add to the combined Parts A and B and mix bystirring to form the aqueous coating solution. Apply the coatingsolution to a suitable substrate, such as paper or a metal sheet, of anelectrophotographic recording element, as by flow-coating, and dry thecoated substrate at about C. for about 2 minutes. During the heating anddrying period, the encapsulant (Pliolite S-SE) of the EDSZO and theresin (Piccotex 131-119) of the aqueous photoconductive coating solutiontend to fuse together to form the photoconductive layer of theelectrophotographic recording element. The quantities of the ingredientsof Example 3 may be varied so that the ratio of zinc oxide to the totalquantity of resin (Pliolite S-SE and Piccotex 131419) in the aqueouscoating solution is within the range of 23:1 by weight. Thephotoconductive layer so formed is superior in its sensitivity tovisible light to photoconductive layers of the prior art formed fromaqueous coating solutions.

The electrophotographic recording element so produced may be processedin the usual manner for producing electrophotographic images. Forexample, the electrophotographic recording element is first madesensitive to light by applying an electrostatic charge to thephotoconductive layer in the dark. The electrostatic charge may beapplied by any means known in the art, such as by a corona dischargedevice. The photoconductive layer is then exposed to a light image todischarge it selectively and to provide a conductivity pattern, that is,a latent image, thereon. The latent image may be developed by applying apigmented resin powder of suitable electrostatic charge to the exposedphotoconductive layer.

Another aqueous coating solution, employing EDSZO, for coating theconductive substrate of an electrophotographic recording element with animproved U-3101 UBATOL (14% solids by weight, water suspensions andacrylato polymer resin).

80.0 U.B.S. Chemical 00.,

Cambridge, Mass.

EXAMPLE 4.Conltinued PART B Weight in grams Durez 15546 (Rosin modified12.5

polyester, a resin of 14% solids in an alkali-soluble solution).

Ingredients Supplier Durez Plastics and Chemical Corp, North End,Tonawanda, N.Y.

Part B is prepared as follows: Add 1 part 26 B. of ammonia to 43.6 partsby weight of cold water. Stir in 7.4 parts by weight of alkali-solubleresin (Durez 15546) using moderate but thorough agitation. Heat and add1 part by weight of ammonia (26 B.) until solution is complete. Keep thesolution cool until used.

PART

Ingredients Weight in grams Polyethylene emulsion (a resin, 14% solidsin an aqueous emulsion).

PART D I Weight Ingredients in Supplier grains Igepal-Qo-QQO at aqueous0. 6 Antara Chemical Ino.,

solutlon (nonylperoxypoly New York, N .Y.

(ethyleneoxy) ethanol, a

surfactant). KP-MO (tnbutoxy ethyl phos- 0.7 Food Machinery Corp.,

phate, plasticizer and leveling agent).

Make a mixture of the ingredients of Parts A, B, and C, adding theingredients in the order indicated and using mild agitation forblending. Mix the ingredients of Part D with those of the blended PartsA, B, and C for at least 30 minutes to form a homogeneous mixture.

Add in the proportions of 7 g. of EDSZO, 5 ml. of water, and 5 ml. ofthe homogeneous mixture of Parts A, B, C, and D and mix until the EDSZOparticles are evenly dispersed in the mixture, thus forming the desiredaqueous photoconductive coating solution. This coating solution may beused to flow-coat any suitable conductive substrate to apply theimproved photoconductive layer thereto. The quantities of theingredients of Example 4 may be varied, as desired, to provide anaqueous coating solution in which the ratio of zinc oxide to the totalquantity of resin in the solution is 23:1 by Weight, (Zinc oxideztotalresin).

Another substantially aqueous photoconductive coating solution havingEDSZO particles dispersed therein may be made, in accordance with thepresent invention as follows:

New York, N .Y.

EXAMPLE 5 Component- Percent by weight Carnauba Wax 22 Stearic acid 2Diethanolamine 1 Water 75 The diethanolamine is dissolved in the waterand heated to 200-210 F. The stearic acid is dissolved in the wax atZOO-210 F. The hot Wax solution is poured slowly into the hot watersolution While maintaining vigorous agitation. The agitation iscontinued until the solution becomes smooth and homogeneous.Proportionately, 5 g. of EDSZO, prepared in accordance with Example 2,is dispersed in 10 ml. of the aforementioned homogeneous solution(emulsion) to form the aqueous photoconductive coating solution. Thisquantities of the ingredients in Example 5 may be varied so that theratio of zinc oxide to the total quantity of resin (carnauba wax andPliolite S-SE) in the coating solution is from 2-3z1 by weight, (zincoxideztotal resin in coating solution). This coating solution can beapplied to any suitable substrate, as by flow-coating, as described forthe aqueous photoconductive coating solution of Example 3.

Each of the aqueous coating solutions described in Examples 3, 4, and 5is essentially a dispersion of EDSZO in an aqueous solution (mixture,dispersion, or emulsion) of a resinous binding agent for the EDSZO. Theaqueous coating solutions described in Examples 3, 4, and 5 differ fromeach other primarily in the type of resinous binding agent (naturaland/or synthetic) used and in the type of wetting agents, levelingagents, and surfactants employed. Since the photoconductivedyesensitized zinc oxide herein described is encapsulated, the EDSZO maybe dispersed in a large number of aqueous mixtures of resinous bindingagents to provide an improved aqueous coating solution whereindesorption of the light-sensitizing dye into the Water vehicle of thecoating solution is substantially prevented.

From the foregoing description, it can be seen that there has beenprovided improved photoconductive layers that can be deposited onsuitable substrates from improved aqueous coating solutions. Although,to the naked eye, the photoconductive layers produced by the improvedphotoconductive coating solutions and methods herein described aresomewhat similar in appearance to those of the prior art, they aresuperior in their lightresponsive characteristics to any derived fromthe aqueous coating solutions and methods of the prior art. Hence, theimproved photoconductive layers can be described best by their method ofmanufacture.

Although the improved products and methods have been described with alimited number of examples, variations in the products and methods, all.coming within the spirit of the invention, will, no doubt, readilysuggest themselves to those skilled in the art. Hence, it is desiredthat the foregoing shall the considered. as illustrative and notlimiting.

What is claimed is:

1. An aqueous coating solution for forming a photoconductive layer on asubstrate, said coating solution comprising particles of aresin-encapsulated dye-sensitized photoconductive zinc oxide dispersedin an aqueous mixture comprising a resinous binding agent, thesensitizing dye of said particles being water soluble, and theencapsulating resin of said particles being water insoluble.

2. An aqueous coating solution for forming a photoconductive layer on asubstrate, said coating solution comprising particles of aresin-encapsulated dye-sensitized zinc oxide dispersed in an aqueousmixture comprising a resinous binding agent, the ratio of zinc oxide tothe total quantity of the resin of said encapsulant and said bindingagent in said coating solution being within a range of about 2 to 3:1 byweight, the sensitizing dye of said particles being water soluble, andthe encapsulating resin of said particles being water insoluble.

3. An aqueous coating solution for forming a photoconductive layer on asubstrate of an electrophotographic recording element, said coatingsolution comprising particles of encapsulated dye-sensitized zinc oxidedispersed in an aqueous mixture comprising a resinous binding agent,each of said particles comprising a core of photoconductive zinc oxide,

at least one water soluble light-sensitizing dye adsorbed to said core,and

a substantially water insoluble resin encapsulating said dyed core.

4. An aqueous coating solution for forming a photoconductive layer of anelectrophotographic recording element, said coating solution comprisingparticles of encapsulated dye-sensitized zinc oxide dispersed in anaqueous mixture comprising a resin, each of said particles comprising acore of photoconductive zinc oxide,

at least one water soluble light-sensitizing dye adsorbed to said core,and

a substantially water insoluble resin encapsulating said dyed core, eachof said particles having a diameter of about 1 micron.

5. The method of storming a photoconductive layer on a substrate of anelectrophotographic recording element comprising the steps of mixingtogether zinc oxide particles, a water soluble light-sensitizing dye, aresin substantially insoluble in water, and a solvent for said resin,whereby to dye-sensitize said zinc oxide particles and coat thesensitized particles with resin,

drying the coated particles to provide resin-encapsulated,dye-sensitized, photoconductive zinc oxide particles,

dispersing said encapsulated particles in water to form an aqueouscoating solution, and

coating said substrate with said coating solution to form saidphotoconductive layer thereon. 6. A method of forming a photoconductivelayer on a substrate of an electrophotographic recording element, saidlayer comprising particles of resin-encapsulated dyesensitized zincoxide wherein each particle comprises a core of zinc oxide, a watersoluble dye adsorbed to said core, and a water soluble resinencapsulating said dyed core, said method comprising dispersing saidparticles of resin-encapsulated dyesensitized zinc oxide particleshomogeneously in an aqueous mixture comprising a resinous binding agent,whereby to form an aqueous coating solution, the ratio of zinc oxide tothe total quantity of resin encapsulant and said resinous binding agentin said coating solution being within a range of about 2 to 3:1, byweight,

applying said coating solution to said substrate to coat said substrate,whereby to form said photoconductive layer thereon, and

heating said substrate and said coating solution applied thereto to fusesaid resin-encapsulated dye-sensitized zinc oxide particles with saidresinous binding agent, whereby to dry said photoconductive layer.

7. A method of forming a photoconductive layer on a substrate of anelectrophotographic recording element, said method comprising mixingtogether in a homogeneous mixture photoconductive zinc oxide particles,at least one water soluble light-sensitizing dye, a resin substantiallyinsoluble in water, and a solvent for said resin,

drying said homogeneous mixture to form particles of resin-encapsulated,dye-sensitized photoconductive zinc oxide,

dispersing said particles of resin-encapsulated dye sensitized zincoxide in water containing a resin binding agent to form a homogeneousaqueous coatng solution,

applying said coating solution to said substrate to coat said substrate,whereby to form said photoconductive layer thereon, and

heating said photoconductive layer to fuse said resinencapsulatedparticles with said resinous binding agent.

.8. A method of forming a photoconductive layer on a substrate of anelectrophotographic recording element, said method comprising making ahomogeneous mixture of resin-encapsulated dye sensitized zinc oxideparticles dispersed in an aqueous mixture comprising a resinous bindingagent, whereby to form an aqueous coating solution, flow coating saidcoating solution on said substrate to coat said substrate, whereby toform said photoconductive layer thereon, and drying said substrate andsaid photoconductive layer thereon, said particles comprising a core ofphotoconductive zinc oxide, at least one water soluble light-sensitizingdye adsorbed to said core, and a substantially water insoluble resinencapsulating said dyed core. 9. A method of forming a photoconductivelayer on a substrate of an electrophotographic recording element, saidmethod comprising mixing together in a homogeneous mixturephotoconductive zinc oxide, at least one water soluble lightsensitizingdye, a resin substantially insoluble in water, and a solvent for saidresin, spray drying said homogeneous mixture whereby to form particlesof resin-encapsulated, dyed, photoconductive Zinc oxide, the ratio ofsaid zinc oxide to said resin being in the range between 2 and 7.5 :1 byweight, dispersing said particles of resin-encapsulated dyedphotoconductive zinc oxide in an aqueous mixture comprising a resinousbinding agent, whereby to form a homogeneous aqueous coating solution,applying said coating solution to said substrate to coat said substrate,whereby to form said photoconductive layer thereon, and heating saidphotoconductive layer to fuse said resinencapsulated particles 'withsaid resinous binding agent. 10. A method of forming a photoconductivelayer on a substrate of an electrophotoi'graphic recording element, saidmethod comprising making a homogeneous mixture of resin encapsulateddye-sensitized zinc oxide particles dispersed in an aqueous mixturecomprising a resinous binding agent, whereby to form an aqueous coatingsolution, flow coating said coating solution on said substrate to coatsaid substrate, whereby to form said photoconductive layer thereon,drying said substrate and said photoconductive layer thereon, saidparticles comprising a core of photoconductive zinc oxide, at least onewater soluble light-sensitizing dye adsorbed to said core, and asubstantially water insoluble resin encapsulating said dyed core, theratio of zinc oxide to the total quantity of resin in said coatingsolution being in the range of about 2 to 3: 1, by weight. 11. A methodof forming a photoconductive layer on a substrate of anelectrophotographic recording element, from a substantially aqueousdispersion of particles wherein each particle comprises a core of zincoxide, a water soluble dye adsorbed to said core, and a water insolubleresin encapsulating said dyed core, said method comprising making ahomogeneous dispersion in the proportions of 7 g. of resin-encapsulateddye-sensitized zinc oxide particles, 5 ml. of water, and 5 ml. of ahomogeneous mixture of g. of a 14% acrylate polymer resin watersuspension, 12.5 g. of a 14% rosin modified polyester alkali-solublesolution, 7.5 g. of a 14% polyethylene aqueous emulsion, 0.6 g. of a 10%aqueous solution of nonylperoxypoly(ethyleneoxy)ethanol, and 0.7 g. oftributoxy ethyl phosphate, applying said homogeneous dispersion to saidsub- 3,428,452 9 10 strate, whereby to form said photoconductive layer,14. A method of forming a photoconductive layer on and a substrate of anelectrophotographic recording element, drying said photoconductivelayer. said method comprising 12. A method of forming a photoconductivelayer on mixing together in a homogeneous mixture in the proa substrateof electrophotographic recording element, portions of 454 g. ofphotoconductive zinc oxide, 2 said method comprising making ahomogeneous dispersion in the proportions 5 parts of resin encapsulateddye-sensitized zinc oxide particles, and

sensitized zinc oxide in an aqueous mixture comprising a resinousbinding agent, whereby to form an aqueous coating solution, the totalquantity of resinous binding agent in said coating solution being suchas to provide, with said resin encapsulant, a ratio of 1 part of resinto about 2 to 3 parts of zinc oxide by weight,

applying said coatinlg agent to said substrate, whereby to form saidphotoconductive layer, and

drying said photoconductive layer.

ml. of a water soluble light-sensitizing dye comprising 1.2 g. offluorescein disodium salt and 3.6 g. of Brilliant Blue FCF (Cl. 42090)in 100 ml. of methanol, 120 g. of a high styrene butadiene polymerresin, and 800 ml. of toluene,

ml. of about 22% wax emulsion in water, 10 drying and forming a powderof said homogeneous applying said homogeneous dispersion to saidsubmixture, whereby to form particles of resin encapsustrate, whereby tocoat it and to form said photolated dye-sensitized photoconducti've zincoxide, the conductive layer, and ratio of said zinc oxide to said resinbeing between drying said photoconductive layer, each of said resin 2and 7.5 :1 by weight,

encapsulated dye-sensitized zinc oxide particles comdispersing saidparticles of resin encapsulated dyeprising sensitized zinc oxide in anaqueous mixture coma core of photoconductive zinc oxide, prising aresinous binding agent, whereby to form a water solublelight-sensitizing dye adsorbed thereto, an aqueous coating solution, thetotal quantity of and resinous binding agent in said coating solutionbeing a substantially water insoluble resin surrounding said such as toprovide, with said resin encapsulant, a dyed core. ratio of 1 part ofresin to about 2 to 3 parts of zinc 13. A method of forming aphotoconductive layer on oxide, by 'weight, a substrate of anelectrophotographic recording element, applying said coating agent tosaid substrate, whereby said method comprising to form saidphotoconductive layer, and

mixing together in a homogeneous mixture photocondrying saidphotoconductive layer.

ductive zinc oxide, at least one water-soluble lightsensitizing dye, aresin substantially insoluble in References Cited water, and a solventfor said resin, UNITED STATES PATENTS spray drying said homogeneousmixture whereby to form particles of resin encapsulated dyed photocon-3,060,021 10/1962 GrPllg ductive zinc oxide, the ratio of said zincoxide to 3,121,005 2/1964 lMlddleton et said resin being in the rangebetween 2 and 7.521 by 3,198,647 8/1964 'F X 'weight, 3,212,887 10/1965Miller et a1. 96--1.2 dispersing said particles of resin encapsulatedd-ye- 3,238,149 3/1966 p 252501 FOREIGN PATENTS 888,371- 1/1962 GreatBritain.

J. TRAVIS BROWN, Primary Examiner.

C. E. VAN HORN, Assistant Examiner.

US. Cl. X.R.

